This invention relates to Faraday rotators and particularly Faraday isolators for use with high power lasers. Faraday isolators capable of withstanding high power are required to decouple high power laser sources from reflective targets. Reflections can damage the laser and/or cause it to operate erratically. Lasers used in industrial metalworking applications are particularly prone to reflections from metal work-pieces. For example, metal targets can couple nearly 100% of the incident radiation back into industrial 1 μm laser sources.
Optical fiber is particularly useful for delivering high power laser radiation in industrial environments. It is sealed from dust and other contaminants. It is flexible, durable and lightweight. Fiber lasers are particularly efficient laser sources for high power industrial use. A fiber laser can be readily coupled to an optical fiber. It is therefore desirable to mate optical fiber to high power Faraday isolators.
Laser radiation guided within optical fiber is converted into a free space beam when used with a Faraday isolator. This is often achieved by allowing laser radiation emanating from the core of an optical fiber termination to diverge to the desired size and then collimating this radiation into a beam with a lens. Such a combination of elements is called a fiber collimator. The free space beam out of a first fiber collimator is then transmitted through a Faraday isolator where it may be further directed onto a target directly or it may be re-coupled back into a second fiber collimator for fiber beam delivery. Faraday isolators used with one or more fiber collimators are often described as fiber pigtailed Faraday isolators.
Faraday isolators which are fiber pigtailed on the input and output have been widely used for low power applications, such as fiber optic telecommunications. In such devices radiation transmitted in the forward direction which does not couple into the core of the output fiber is of such low power that it cannot damage anything in the fiber collimator structure. With high power lasers however, the optical radiation (e.g., IR light) which does not couple into the core of the output collimator either couples into the surrounding cladding layer, from which it ultimately diverges and readily burns the optical fiber polymer protective coating layer, thereby destroying the fiber, or it destroys the fiber collimator if it strikes any adhesives which hold the fiber in alignment. This is a practical problem for fiber pigtailed high power Faraday isolators because Faraday isolators are used at wavelengths near an absorption peak where there is always a residual absorption. This is done because Faraday Rotation greatly increases near an absorption, allowing the Faraday isolator to be smaller and less expensive. This residual absorption leads to a temperature gradient across the beam profile within the Faraday optical material, the magnitude of which increases with increasing laser power. This thermal profile leads to two thermal effects within the Faraday optic: thermal lensing and thermal birefringence. For substantially round laser beams, the thermal gradient is parabolic and acts, to first order, as a spherical lens. Increasing the beam size through the Faraday optic can reduce this thermal lensing, but it does so at the expense of making the device bulkier. The thermal lens changes the beam parameters such that optical radiation from a first fiber collimator does not completely couple into the core of a second fiber collimator at all laser operating powers. Thermal birefringence changes polarization states emanating from a first (input) fiber collimator as they propagate through the Faraday isolator. These changed polarization states will also not couple efficiently into the core of a second (output) fiber collimator. Therefore, a structure is desired which does not lead to damage in the presence of thermal effects in a high power fiber pigtailed Faraday isolator.
There are two types of Faraday isolators used with lasers: Polarization Maintaining (“PM”) isolators and Polarization Insensitive (“PI”) isolators. PM isolators are used with laser sources which have output radiation that is polarized. They require linear polarizers at the input and output joining a 45° Faraday Rotator. Fiber pigtailed PM isolators have been used with PM fiber which maintains the launched linear polarization into, and out of, the linear polarizers of the PM Faraday isolator. For low power operation the linear polarizers can be absorptive type polarizers such as Corning Polarcor™ polarizer (Corning Glass Works of Corning, N.Y.) or scattering/mode stripping fiber types exemplified by Corning Single Polarization Fiber or Crystal Fibre LMA PZ 20 Large Mode Area Polarizing Fiber (Crystal Fibre of Birkerod, Denmark). Polarcor is a thin (typically 0.5 mm) glass plate polarizer that absorbs any polarization component orthogonal to the polarization transmission axis. Corning Single Polarization Fiber, for example, rejects any component of radiation polarized orthogonal to the transmission axis into the cladding where it is ultimately stripped in the polymer protective coating. Typically lengths of one hundred centimeters or more are required to ensure highly linear polarization. Because it can be fusion spliced to commonly used PM fiber, this structure is of a form that is convenient for robust, compact, low power isolators.
However, these types of polarizers function by absorbing or scattering the undesired polarization component. Thus, they are readily prone to failure if the same designs are employed for high power applications. In contrast, for high power operation, the linear polarizers are typically polarizing beam-splitter cubes or Brewster-angle thin film polarizers. Any polarization component orthogonal to the transmission axis of such polarizers is angularly rejected away from the beam path. Because the rejected polarization means that energy is directed away from the beam path, such polarizers are located outside of the magnet body so that the rejected radiation can be directed onto a beam dump. This design has in the past required that the size of the Faraday isolator be even larger than for low power applications. A robust, compact, fiber pigtailed Polarization Maintaining Faraday isolator suitable for use with high power lasers is desired.
Polarization Insensitive Faraday isolators are useful with lasers sources which are unpolarized or randomly polarized. There are two common ways to construct PI isolators. The first approach, as for example disclosed in U.S. Pat. No. 4,178,073, is the use of a birefringent crystal beam displacer to split a laser beam into two distinct beams of orthogonal polarization. In operation the device transmits the beams first through 45° of non-reciprocal Faraday rotation and then through 45° of reciprocal optical rotation (using a quartz rotator or waveplate), and finally recombines the two beams in a second beam displacer. A disadvantage of this approach is that the effective aperture size of the Faraday isolator is typically doubled in order to transmit both orthogonally polarized beams. The result is that an already bulky magnetic encasement must be even larger. Furthermore, common birefringent beam displacers have lengths which are approximately ten times as large as the laser beam diameter. For high power laser application where it is desired to increase the laser beam size to reduce thermal lensing, the indicated size of such beam displacers is very large and difficult to fabricate with high optical quality. Even so, such bulky designs have found wide usage in industrial settings with fiber pigtailing because any reflected power back into the isolator is blocked by physical beam blocks rather than being coupled back into the input fiber collimator where damage could occur.
The second basic approach, which is used to achieve Polarization Independent Faraday Isolation as disclosed in U.S. Pat. No. 4,548,478, is to use birefringent wedges. In a common design, laser light from an input fiber collimator is directed through an input birefringent wedge. The orthogonally polarized birefringent axes define two different refractive indices which refract the two polarizations into slightly different angles. The 45° of Faraday rotation then rotates the polarization axis into that of a second, output birefringent wedge if the second wedge also has its birefringent axis rotated 45° to the input wedge polarization axis. In such a PI isolator, if a second wedge is also oriented at the output in a flipped mirror image to the input wedge about a reflection axis through the middle of the Faraday isolator, then the orthogonally polarized beams exit the output wedge in parallel with one another, but displaced by a small amount, with low transmission loss. With such an approach any reflected light back into the Faraday isolator is given an additional, non-reciprocal, 45° of Faraday rotation such that the backward propagating polarization states are rotated 90° at the input birefringent wedge. The input wedge then refracts the two backward propagating polarization states away from the input beam path by a small angle (on the order of 1-2°).
Low power fiber pigtailed isolators used for telecom applications in the wavelength range of 1.5 μm typically use highly efficient, low absorption Faraday rotators such as Bismuth Iron Garnet (“BIG”) that can achieve 45° of Faraday rotation in a few millimeters or less. Because the Faraday rotation occurs in such a short length, the small angular deviation through the “BIG” optic after the first birefringent wedge (on the order of 4°) does not require the aperture size of the Faraday isolator to significantly increase beyond the beam size. For this reason birefringent wedge based PI isolator designs are more compact and less expensive than beam displacer based PI isolator designs at telecom wavelengths. Fiber pigtailed PI isolators using birefringent wedges have been the dominant basic form used at low powers.
However, at typical high power laser wavelengths, 1 μm for example, Terbium Gallium Garnet (“TGG”) is a common Faraday optic material. TGG is a much less efficient Faraday optic material than BIG, which cannot be used at such wavelengths due to a strong absorption. For high magnetic fields, on the order of 10,000 gauss, approximately 2 cm of TGG is required for 45° rotation at 1065 nm. In this case, the small angular deviation in birefringent wedge-based PI isolator designs through the approximately 2 cm TGG optic requires the magnet aperture size to be increased. This significantly increases the size of the overall package. It is further desired to have the original input beam path restored after traversing all optics to simplify alignment and maximize coupling efficiency from fiber to fiber. Additionally, in numerous instances it is desired to have a fiber pigtailed input and a free space output beam. In such usage, it is desired to have the two polarization components of the PI isolator perfectly overlapped at the output in order to achieve minimum focused spot size at the workpiece. Hosokawa in U.S. Pat. No. 5,408,354 discloses the use of a birefringent crystal plane plate beam displacer to bring the two polarization component beams out of first and second birefringent elements to converge the two polarization components back into coincidence. Although the output beam path as disclosed is parallel to the input beam path, it is however displaced from the input beam axis—which causes the magnet aperture size to increase. All of the factors listed above increase the magnet aperture size of birefringent wedge based designs when used with high power Faraday optic materials. This diminishes the potential size and cost benefits which birefringent wedge based PI isolator designs have over a bulky beam displacer based approach. A birefringent wedge based fiber pigtailed PI isolator design for high power applications which maintains the beam path through the Faraday isolator and does not require the magnet aperture size to increase is desired.
In fiber pigtailed PI isolators in lower power applications, the isolated power directed into the cladding layer and stripped by the polymer coating is well below the threshold for material damage. However at average power levels as low as 10 W, such isolated power can damage the protective polymer coating of the input fiber. Thus, fiber pigtailed birefringent wedge based PI isolators have not been considered suitable for high-power applications.
What is needed is a compact Faraday isolator for high power applications, and in particular an inherently small PI Faraday isolator, such as a fiber pigtailed Faraday isolator that does not require an enlarged clear aperture and resists internal damage at high power.